A. Field of the Invention
The present invention is directed toward the field of data capture and area detection circuitry. More particularly, the present invention is directed toward the field of data capture and area detection circuitry for use in disk drive servo channels.
B. Description of the Related Art
When performing data capture operations based on the area detection of an incoming signal, data capture inaccuracies are caused by DC shifts and dropouts that occur in the incoming signal. One example of a system in which DC shifts and signal dropout cause data capture inaccuracies is a disk drive.
In a disk drive, servo signals are written into servo fields on the tracks of a recording medium. The servo signals identify the track on which they are written and the location of the servo signals on the track. This information is used during the operation of a disk drive to record and retrieve data and position the head.
Typically, servo signals are stored on a recording medium as periodic signals having amplitude and frequency, such as a sine wave. FIG. 1 illustrates a sine wave 100 in which each cycle represents a character 101.sub.1-3. The value for each character 101.sub.1-3 is represented by the total area under a rectified sine wave for a cycle corresponding to the character 101.sub.1-3. A servo signal word is represented by the sum of the area values for a set of characters.
In determining the value for a character 101.sub.1-3, a traditional character detection circuit first takes amplitude samples of points on a sine wave 100 cycle corresponding to the character 101.sub.1-3. The character detection circuit then sums the absolute value of each amplitude sample to obtain a character value. A traditional word detection circuit receives the character values and accumulates them to obtain a word value.
When a servo signal experiences a DC shift, the character value determined by a traditional character detection circuit is incorrect. In a disk drive, there are two major sources of DC shift. One source is the magneto-resistive heads that are employed for exchanging data with the recording medium. Another cause is thermal asperities, which occur when the spacing between a disk drive's recording medium and read-write head falls below a certain threshold.
FIG. 2(a) illustrates the servo signal sine wave 100 shown in FIG. 1 having a DC shift 102. Using a traditional character detection circuit, the magnitude of the shift 102 is added into every sample of the sine wave cycle. As a result, the character value is inflated by an amount equal to the number of amplitude samples multiplied by the magnitude of the DC shift 102.
When a servo signal experience a dropout, the resulting character value will cause the word value to be incorrect. Dropouts are often caused in a disk drive by defects in the recording medium. FIG. 2(b) illustrates the servo signal sine wave 100 from FIG. 1 having a dropout 103 in the second character 101.sub.2. The dropout 103 will cause a character detection circuit to provide a character value that is substantially less than the desired character value. As a result, the word value, which is derived from a sum of character values, will be significantly attenuated.
Accordingly, it is desirable for a data capture circuit to compensate for DC shifts that occur in signals. It is also desirable for compensation to be provided for the occurrence of signal dropout.